Multiple-core planar optical waveguides and methods of fabrication and use thereof

ABSTRACT

A multiple-core optical waveguide comprises: a substrate; lower and upper waveguide core layers; a waveguide core between the upper and lower waveguide core layers; upper and lower cladding; and middle cladding between the upper and lower waveguide core layers substantially surrounding the waveguide core. Each of the lower, middle, and upper claddings has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core. Along at least a given portion of the optical waveguide, the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of a propagating optical mode supported by the optical waveguide, the lateral extent of the supported optical mode being determined at least in part by the width of the waveguide core along the given portion of the optical waveguide.

Subject matter disclosed in this application may be related to subjectmatter disclosed in: i) U.S. non-provisional application Ser. No.10/836,641 filed Apr. 29, 2004 (U.S. Pat. App. Pub. No. 2004/0264905 A1published Dec. 30, 2004); ii) U.S. non-provisional application Ser. No.10/682,768 filed Oct. 9, 2003 (not yet published); iii) U.S.non-provisional App. Ser. No. 10/661,709 filed Sep. 12, 2003 (U.S. Pat.App. Pub. No. 2004/0129935 A1 published Jul. 8, 2004); and iv) U.S.non-provisional application Ser. No. 10/609,018 filed Jun. 27, 2003(U.S. Pat. App. Pub. No. 2004/0052467 A1 published Mar. 18, 2004). Eachof said non-provisional applications is hereby incorporated by referenceas if fully set forth herein.

BACKGROUND

The field of the present invention relates to optical waveguides. Inparticular, multiple-core planar optical waveguides are disclosedherein, as well as methods of fabrication and use thereof.

Planar optical waveguides fabricated on waveguide substrates may beincorporated into a variety of optical assemblies. Such opticalwaveguides may be fabricated with multiple cores or core layers. The useof such multiple-core planar optical waveguides may be advantageous in avariety of ways, as set forth hereinbelow.

SUMMARY

A multiple-core optical waveguide comprises: a substantially planarwaveguide substrate; a lower waveguide core layer; an upper waveguidecore layer; a waveguide core between the upper and lower waveguide corelayers; lower cladding between the substrate and the lower waveguidecore layer; upper cladding above the upper waveguide core layer; andmiddle cladding between the upper and lower waveguide core layerssubstantially surrounding the waveguide core. Each of the lower, middle,and upper claddings has a refractive index less than refractive indicesof the lower waveguide core layer, the upper waveguide core layer, andthe waveguide core. Along at least a given portion of the opticalwaveguide, the upper and lower waveguide core layers extend bilaterallysubstantially beyond the lateral extent of a propagating optical modesupported by the optical waveguide, the lateral extent of the supportedoptical mode being determined at least in part by the width of thewaveguide core along the given portion of the optical waveguide. Theoptical waveguide may further comprise a second waveguide core. Thewaveguide cores may taper in various ways so as to effect modeconversions by optical coupling between the waveguide cores. Thewaveguide may terminate at an end face thereof for optical end-couplingwith an optical fiber or with a planar waveguide, and a terminal segmentof the waveguide may be adapted for such end-coupling.

The waveguide may be fabricated by: forming a lower cladding layer on awaveguide substrate; forming a lower waveguide core layer on the lowercladding layer; forming a lower portion of a middle cladding layer onthe lower core layer; forming a waveguide core on the lower portion ofthe middle cladding layer; forming an upper portion of the middlecladding layer over the waveguide core and on exposed areas of the lowerportion of the middle cladding layer; forming an upper waveguide corelayer on the upper portion of the middle cladding layer; and forming anupper cladding layer on the upper waveguide core layer. Spatialpatterning of various waveguide cores, core layers, or claddings may bedone sequentially or concurrently.

Objects and advantages pertaining to multiple-core planar opticalwaveguides as disclosed herein may become apparent upon referring to thedisclosed exemplary embodiments as illustrated in the drawings anddisclosed in the following written description or claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are cross-sectional views of exemplary multiple-core opticalwaveguides.

FIGS. 2A-2E are cross-sectional views of an exemplary multiple-coreoptical waveguides.

FIGS. 3A-3E are plan and cross-sectional views of an exemplarymultiple-core optical waveguide.

FIGS. 4A-4D are plan and cross-sectional views of an exemplarymultiple-core optical waveguide.

The embodiments shown in the Figures are exemplary, and should not beconstrued as limiting the scope of the present disclosure and/orappended claims. It should be noted that the relative sizes orproportions of structures shown in the Figures may in some instances bedistorted to facilitate illustration of the disclosed embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of a multiple-core low-contrast planar waveguideare shown in cross-section in FIGS. 1A-1E positioned on a waveguidesubstrate 102. Substrate 102 may comprise a semiconductor substrate suchas silicon in this example, although any suitable substrate material maybe employed. In this example, low-contrast waveguide core 113 comprisesdoped silica and is surrounded by lower-index middle cladding layer 120b, which comprises doped or undoped silica with a refractive indexbetween about 1.44 and 1.46. The terms “low-contrast” or“low-index-contrast” as used herein shall denote index contrast lessthan about 5%. The index contrast between waveguide core 113 and middlecladding 120 b in this example is less than about 5%, or may be betweenabout 0.5% and about 3%, or may be between about 1% and about 2%. Forexample, core 113 may have an index typically between about 1.46 andabout 1.48. Middle cladding layer 120 b and core 113 therein aredisposed between two doped silica core layers 111 and 112, which are inturn disposed between lower-index upper cladding 120 c and lower-indexlower cladding 120 a. Cladding layers 120 a and 120 c in this examplecomprise doped or undoped silica with refractive indices similar to orthe same as middle cladding 120 b. Core layers 111 and 112 may comprisedoped silica, with refractive indices larger than those of the claddinglayers 120 a, 120 b, and 120 c, and similar to or the same as therefractive index of waveguide core 113. Any other suitable materials maybe employed for forming core 113, core layers 111 and 112, or claddings120 a, 120 b, or 120 c.

In the examples of FIGS. 1B and 1E, the core layers 111 and 112 extendbilaterally substantially beyond the transverse extent of a propagatingoptical mode supported by the waveguide. Such an optical mode istypically confined laterally by the waveguide core 113, and thetransverse extent of core 113 at least in part determines the transverseextent of the supported optical mode. In the examples of FIGS. 1A, 1C,and 1D, claddings layers 120 a, 120 b, or 120 c may be formed so as toyield a protruding lateral surface terminating core layer(s) 111 and/or112. Such lateral surfaces may be provided at varying depths, and may ormay not extend downward near or beyond the depth of core 113. Awaveguide may be formed to include multiple segments having various ofthe configurations shown in FIG. 1A-1E. In some embodiments having oneor two lateral protruding surfaces, core layers 111 and 112 may extendbilaterally substantially beyond the transverse extent of a supportedoptical mode. Alternatively, in other embodiments the transverse extentof a supported optical mode may be in part determined by the transverseextent of the core layers 111 and 112 (if they terminate sufficientlyclose to waveguide core 113), or by the presence of the lateralprotruding surface (if it is formed sufficiently close to waveguide core113). In all of the exemplary multiple-core low-contrast waveguides ofFIGS. 1A-1E, a supported propagating optical mode is substantiallyconfined vertically by core layers 111 and 112, while the presence ofcore 113 influences the details of the spatial mode profile (along thevertical dimension) near its center.

In an exemplary multiple-core low-contrast waveguide with silica ordoped silica core, core layers, and claddings as described above,waveguide core 113 may be about 0.7 μm thick by about 8 μm wide, corelayers 111 and 112 may each be about 0.6 μm thick, and the thickness ofmiddle cladding 1120 b separating core 113 from each of the core layers111 and 112 may be about 1.5 μm. At a wavelength of about 1.3-1.5 μm,these dimensions may yield a transverse mode size of around 8 μm high byaround 10 μm wide (mode sizes expressed as 1/e² HW power). Otherdimensions or indices for the core, core layers, or claddings may bechosen to yield suitable mode size and shape within the scope of thepresent disclosure. A suitable mode size and shape may be chosen forspatial-mode matching with an optical fiber mode or a mode of anotheroptical waveguide, for example, thereby enabling end-coupling withreduced level of diffractive optical loss. Core 113 may range from about0.3 μm thick up to about 1 μm thick, and between about 3 μm wide andabout 12 μm wide. In some instances where single-mode behavior is notrequired, core 113 may be as wide as about 15 μm or about 20 μm. Corelayers 111 and 112 may range from about 0.3 μm thick up to about 2 μmthick. Refractive indices for core 113 and core layers 111/112 typicallyrange between about 1.46 and about 1.48, while that of the claddings 120a/120 b/120 c typically range between about 1.44 and 1.46. Any othersuitable indices may be employed within one or more of theindex-contrast ranges given above. The portions of cladding 120 bseparating core 113 from core layers 111/112 may range between about 1μm thick and about 3 μm thick. Specific combinations of dimensions willdepend on the desired spatial mode characteristics and the particulardegree of index contrast employed. In addition to doped and undopedsilica, other suitable core and cladding materials may be equivalentlyemployed. As in the previous examples, lower cladding layer 120 a belowcore layer 111 may be sufficiently thick so as to reduce orsubstantially eliminate optical leakage from the waveguide intosubstrate 102 (within operationally acceptable limits), or a reflectivecoating between the cladding and the substrate may be employed (asdescribed hereinabove). The lower cladding may be grater than about 5 μmthick, or between about 6 μm thick and about 12 μm thick or betweenabout 8 μm thick and about 10 μm thick. Similarly, upper cladding layer1120 c above upper core layer 112 may be sufficiently thick so as toreduce or substantially eliminate optical leakage through the uppersurface of the waveguide (within operationally acceptable limits) or tosubstantially isolate a supported optical mode from a use environment(within operationally acceptable limits). The upper cladding may begreater than about 5 μm thick, or between about 6 μm thick and about 12μm thick or between about 8 μm thick and about 10 μm thick.

Fabrication of a multiple-core low-contrast waveguide, such as theexamples shown in FIGS. 1A-1E, typically begins with deposition (inorder) of lower cladding 120 a, core layer 111, and a lower portion ofcladding 120 b. Waveguide core 113 is then formed on the substantiallyplanar upper surface of the deposited cladding 120 b, typically byspatially-selective deposition or by substantially uniform depositionfollowed by spatially-selective removal. After forming core 113,additional cladding 120 b is deposited, which may or may not comprisethe same material as that deposited to form the lower portion ofcladding 120 b. If a deposition process having a substantial degree ofconformality is employed, the upper surface of cladding 120 b mayexhibit a raised portion directly above waveguide core 113. Upper corelayer 112 may be deposited directly on such a non-planar claddingsurface, resulting in a corresponding raised portion of core layer 112directly over waveguide core 113. Upper cladding 120 c may be depositedon the non-planar core layer 112, resulting in a correspondingnon-planar upper surface of cladding layer 120 c. The multiple-corelow-contrast waveguide resulting from this exemplary fabricationsequence would resemble the exemplary embodiment shown in FIG. 1B. If adeposition process for cladding 120 b is employed that yields asubstantially flat upper surface regardless of underlying topology, orif a non-planar upper surface of cladding 120 b is substantiallyplanarized prior to deposition of core layer 112 thereon, then theresulting multiple-core waveguide would resemble the exemplaryembodiment shown in FIG. 1E. In either case (planar or non-planar corelayer 112) the resulting waveguide may be further processed to yieldprotruding lateral surfaces as shown in FIGS. 1A, 1C, or 1D, if neededor desired. For relatively low index contrast (less than about 5%, forexample), and sufficiently thin waveguide core 113 (less than about 1μm, for example), a multiple-core waveguide with a non-planar upper corelayer 112 exhibits optical performance characteristics substantiallysimilar to those exhibited by a multiple-core waveguide with asubstantially planar upper core layer 112.

Exemplary multiple-core planar optical waveguides are shown in FIGS.2A-2E that also include a high-contrast core. As in the precedingexamples, substrate 202 may comprise silicon, although any suitablesubstrate material(s) may be employed, and cladding layers 220 a, 220 b,and 220 c may comprise doped or undoped silica of suitable thicknesses(index between about 1.44 and about 1.46), although any suitablecladding material(s) may be employed. In this example, ahigh-index-contrast waveguide core 213 b may comprise a layer of siliconnitride or silicon oxynitride a few tens to a few hundreds of nanometersin thickness, and several microns in width (high-index-contrast, greaterthan about 5%). The high-contrast core 213 b may extend along the entirewaveguide, or may extend along only one or more segments of thewaveguide while being absent from other segments. Low-index-contrastcore 213 a may comprise doped silica about 0.7 μm thick and about 8 μmwide, with a refractive index in this example between about 1.46 andabout 1.48. Low-index-contrast core layers 211 and 212 may also comprisesilica or doped silica about 0.6 μm thick, with refractive indicessimilar to or the same as waveguide core 213 a. In the examples shownwaveguide cores 213 a and 213 b are in contact; embodiments whereincores 213 a and 213 b are separated by cladding material 220 b shallalso fall within the scope of the present disclosure or appended claims.Over portions of the waveguide where waveguide core 213 b is present andwaveguide core 213 a is at least a few tens of microns wide, or wherewaveguide core 213 b is greater than 1 to 2 μm wide, the presence ofwaveguide core 213 a and core layers 211/212 may have little or noeffect on the optical mode characteristics of the waveguide, which aresubstantially determined by the size, shape, and index-contrast on core213 b. Over waveguide segments lacking waveguide core 213 b, waveguidecore 213 a and core layers 211/212 may support an optical mode withcharacteristics substantially determined by their index contrast,dimensions, and relative positions and the index of cladding layers 220a/220 b/220 c. As the size of waveguide core 213 b decreases below about1 to 2 microns in width until it is no longer present, an optical modesupported by the waveguide undergoes a continuous evolution betweenthese two extremes, and various desired mode sizes, shapes, or othercharacteristics may be achieved by appropriate combinations ofdimensions for the waveguide cores 213 a/213 b and layers 211/212.Lateral portions of claddings 220 a/220 b/220 c and layers 211/212 maybe configured in any of the various ways described hereinabove (i.e.with or without a protruding lateral surface), and the waveguide may beformed to include multiple segments having various of suchconfigurations.

The multiple-core structure of the waveguide of FIGS. 2A-2E enables awide array of optical designs for achieving various optical performanceand/or functionality. As described hereinabove, the high-index-contrastcore layer 213 b may be readily adapted for substantiallyspatial-mode-matched optical end coupling with another optical waveguideor semiconductor optical device, or for optical transverse-coupling withanother optical waveguide (substantially adiabatic, substantiallymodal-index-matched, or otherwise), for illuminating a photodetector, orfor other purposes. The low-index-contrast core 213 a and core layers211/212 may be readily adapted for substantially spatial-mode-matchedoptical end-coupling with another optical waveguide or with an opticalfiber, or for enabling insertion of various free-space opticalcomponents between the ends of two such optical waveguides, or for otherpurposes. Such adaptations of waveguide core 213 a and core layers211/212 may include the presence of waveguide core 213 b at a reducedwidth (less than about 1 μm wide, for example) to achieve the desiredmode characteristics. Substantially adiabatic transitions may be madebetween these two distinct waveguide types (high-index-contrast core andlow-index-contrast multiple-core) by spatially selective patterning ofmaterials forming waveguide cores 213 a and 213 b.

As shown in FIGS. 3A-3E, a high-index-contrast waveguide core 313 b afew microns wide may be patterned between substantially uniformlow-index-contrast core layers 311/312 and substantially uniformwaveguide core material layer 313 a over a first segment 300 a of awaveguide. Along this first waveguide segment 300 a, the high-contrastwaveguide core 313 b substantially determines the characteristics of aguided optical mode, while layers 311/312/313 a have negligibleinfluence on the properties of the waveguide (FIG. 3B). Along a secondwaveguide segment 300 b, waveguide core material layer 313 a may bepatterned as well as waveguide core 313 b. Layer 313 b may be patternedto continue the presence of the high-index-contrast core, while layer313 a may be patterned to form the low-index-contrast waveguide core(FIG. 3C). The patterning of layer 313 a may be such that thelow-index-contrast waveguide core appears gradually (i.e., substantiallyadiabatically, as shown), or the low-index-contrast core may appearabruptly (not shown). High-contrast core 313 b continues tosubstantially determine the optical characteristics of the waveguidealong segment 300 b. Along a third segment 300 c of the waveguide (FIG.3D), high-contrast waveguide core 313 b is patterned so as to graduallydecrease in width along the length of the waveguide until it finallyterminates, while low-contrast waveguide core 313 a continues to bepresent along the length of segment 300 c. This tapering ofhigh-contrast waveguide core 313 b is sufficiently gradual so as toallow a substantially adiabatic transition between a waveguide opticalmode characteristic of high-contrast waveguide core 313 b at one end ofsegment 300 c to a waveguide optical mode characteristic of low-contrastwaveguide core 313 a and core layers 311/312 at the other end of segment300 c. A fourth segment 300 d of the waveguide includes onlylow-contrast waveguide core 313 a and core layers 311/312, withouthigh-contrast waveguide core 313 b (FIG. 3E). Instead of terminating,high-contrast waveguide core 313 b may taper to some minimum width (lessthan about 1 μm, for example; not shown) and then remain at that widthalong segment 300 d, in order to achieve desired characteristics for anoptical mode supported by segment 300 d. The exemplary optical waveguideshown in FIGS. 3A-3E may serve as an optical mode converter, withoptical power propagating in either direction.

In variants of the multiple-core embodiments of FIGS. 2A-2E and 3A-3E,the high-contrast core 213 b/313 b may be positioned at any suitablevertical position relative to the low-contrast core layers 211/311 and212/312 and the low-contrast core 313 a. Positioning of thehigh-contrast core 213 b/313 b at one of the low-contrast layerinterfaces may reduce the number of fabrication steps (by eliminatingthe need to deposit a layer surrounding core 213 b/313 b in two steps).Placement of high-contrast core 213 b/313 b in contact with low-contrastcore 213 a/313 a between core layers 211/311 and 212/312 may result inpreferential optical coupling into the lowest-order symmetric modesupported by the multiple-core low-contrasts waveguide. Instead of thesubstantially adiabatic transitions between core high-contrast core 313b and low-contrast core 313 a (FIGS. 3A-3E), in other variousembodiments cores 313 a and 313 b may appear and/or terminate abruptlyinstead of gradually. Such an arrangement may be appropriate forsubstantially modal-index-matched optical transverse-coupling betweenthe cores, instead of substantially adiabatic transverse-coupling. Manyother variants of these embodiments may be contemplated within the scopeof the present disclosure.

The exemplary waveguides of FIGS. 2A-2E and 3A-3E may be fabricated byprocessing sequences similar to that described hereinabove for thefabrication of the exemplary waveguides of FIGS. 1A-1E. Fabrication of awaveguide, such as the examples shown in 2A-2E for example, maytypically begin with deposition (in order) of lower cladding 220 a, corelayer 211, and a lower portion of cladding 220 b. Cores 213 a and 213 bmay then be formed on the substantially planar upper surface of thedeposited cladding 220 b, typically by spatially-selective deposition orby substantially uniform deposition followed by spatially-selectiveremoval. If the latter, both material layers may be deposited beforeeither is spatially-selectively processed. If cores 213 a and 213 b areto be separated by cladding 220 b, an intermediate layer of cladding 220b would be deposited after forming one core and before forming the other(with or without planarizing the upper surface of this intermediatecladding layer, as discussed hereinabove). After cores 213 a and 213 bare formed, additional cladding 220 b is deposited, which may or may notcomprise the same material as that deposited to form the lower portionof cladding 220 b (or the intermediate portion, if any). If a depositionprocess having a substantial degree of conformality is employed, theupper surface of cladding 220 b may exhibit a raised portion directlyabove waveguide cores 213 a and 213 b. Upper core layer 212 may bedeposited directly on such a non-planar cladding surface, resulting in acorresponding raised portion of core layer 212 directly over waveguidecores 213 a and 213 b. Upper cladding 220 c may be deposited on thenon-planar core layer 212, resulting in a corresponding non-planar uppersurface of cladding layer 1220 c. The multiple-core waveguide resultingfrom this exemplary fabrication sequence would resemble the exemplaryembodiment shown in FIG. 2B. If a deposition process for cladding 220 bis employed that yields a substantially flat upper surface regardless ofunderlying topology, or if a non-planar upper surface of cladding 220 bis substantially planarized prior to deposition of core layer 212thereon, then the resulting multiple-core waveguide would resemble theexemplary embodiment shown in FIG. 2E. In either case (planar ornon-planar core layer 212) the resulting waveguide may be furtherprocessed to yield protruding lateral surfaces as shown in FIG. 2A, 2C,or 2D, if needed or desired. For relatively low index contrast (lessthan about 5%), and sufficiently thin waveguide core 213 (less thanabout 1 μm, for example), a multiple-core waveguide with a non-planarupper core layer 212 exhibits optical performance characteristicssubstantially similar to those exhibited by a multiple-core waveguidewith a substantially planar upper core layer 212.

In the exemplary embodiment of FIGS. 4A-4D, a multiple-core waveguideformed on substrate 402 terminates at a V-groove 403 formed on segment400 c of the substrate. An optical fiber (not shown) received inV-groove 403 may be end-coupled to the waveguide. An optical modesupported by segment 400 a of the waveguide may exhibit a somewhatelliptical transverse mode shape elongated in the horizontal dimension.While such a mode might be adequate for end-coupling to anothersimilarly configured waveguide, it might provide less-than-optimalend-coupling to the optical fiber received in groove 403. Terminalsegment 400 b of the waveguide may be adapted for supporting a morenearly symmetric spatial mode at the end face thereof, thereby enhancingend-coupling to an optical fiber received in V-groove 403. One suitableadaptation is shown in FIGS. 4A-4D, where two areas adjacent segment 400b of the waveguide are etched (or otherwise processed) so as to removecore and cladding materials down to the substrateb 402 and to formlateral protruding surfaces 404. The etched areas are arranged so thatnear the end of the waveguide, core layers 411 and 412 terminate nearenough to the lateral edges of waveguide core 413 so that layers 411 and412 at least in part laterally confine the propagating optical mode. Bychoosing a suitable width for layers 411 and 412 at the end of thewaveguide (the choice based in part on the refractive index of anymaterial, such as an embedding medium or encapsulant, that might beemployed subsequently to fill the etched areas), the shape of thepropagating mode may be made to better match that of an optical fiber,so as to provide end-coupling between the waveguide and fiber at orabove an operationally acceptable level. The transition betweenwaveguide segment 400 a and the end of waveguide segment 400 b may bemade substantially adiabatic, if needed or desired, with core layers 411and 412 tapering in width along the waveguide toward the waveguide endface. It is often the case that an index matching substance is depositedbetween the end of the waveguide and the optical fiber, and suchindex-matching material may be employed to fill the etched areas aswell, provided its refractive index is less than the refractive index ofthe core layers 411 and 412, or no greater than the refractive index ofcladding layers 420 a, 420 b, or 420 c. For ease of processing, in someembodiments a thin end wall 405 may be left at the very end of thewaveguide; the wall would include layers 420 a/420 b/420 c and 411/412.Such an end wall may be made sufficiently thin (less than about 10 μm,typically only 2-3 μm) so as not to substantially influence thepropagating optical mode entering or exiting the end face of thewaveguide. Embodiments with or without an end wall shall fall within thescope of the present disclosure or appended claims.

Instead of etching both core layers 411/412 and all claddings 420 a/420b/420 c in a single step so that core layers 411/412 at least partlylaterally confine the propagating optical mode (resulting in a structuresuch as that of FIGS. 4A-4D, for example), core layers 411/412 may beindividually patterned during fabrication of the waveguide so as totaper in width along the waveguide toward the waveguide end face (notshown). As a result of such a fabrication scheme, middle cladding 420 bwould come into contact with the lateral edges of core layer 411, whileupper cladding 420 c would come into contact with the lateral edges ofcore layer 412.

For the typical dimensions and index contrast disclosed hereinabove forexemplary waveguides, it has been observed that the lower and upper corelayers 411/412 contribute to lateral confinement at the waveguide endface if they terminate less than about 15 μm from the respective lateraledges of the core 413. Wider core layers 411/412 at the end face do notappear to provide a substantially degree of confinement. The widths ofthe terminated core layers 411/412 near the waveguide end face may rangefrom about the width of core 413 up to about 30 μm greater than thewidth of core 413, or may range between about 6 μm wider and about 20 μmwider than core 413, or may range between about 8 μm wider and about 12μm wider than core 413.

Instead of a V-groove for receiving an optical fiber for end-coupling,substrate 402 have a second optical waveguide formed thereon andpositioned for end-coupling (not shown). Alternatively, substrate 402may be adapted for receiving a second planar optical waveguide formed ona second substrate and subsequently assembled with substrate 402 forend-coupling (not shown). In either of these alternative scenarios, theterminal segment 400 b of the waveguide may be adapted in any suitablefashion for enabling end-coupling between the waveguides at or above anoperationally acceptable level.

The exemplary embodiments of multiple-core waveguides shown in FIGS.1A-E, 2A-2E, 3A-3B, and 4A-4D, and variants thereof, exhibit many of thedesirable optical properties exhibited by dual-core waveguides disclosedin earlier-cited application Ser. No. 10/836,641. Adjustment of theindex or thickness of the core, core layers, or cladding layers allowsadjustment of spatial properties of the propagating optical modessupported by the multiple-core waveguide. The multiple-core waveguideenables efficient end-coupling with other optical waveguides, includingother planar optical waveguides and optical fibers. The transversedimensions of an optical mode supported by the multiple-core waveguideare typically substantially invariant with wavelength (at least over therange of typical near-infrared telecommunications wavelengths). Thelowest order mode supported by the multiple-core waveguide may besubstantially spatial-mode-matched with another planar waveguide, orwith an optical fiber (particularly if adapted as in FIGS. 4A-4D). Anoptical mode supported by the multiple-core waveguide tends to exhibit aminimum phase front curvature upon propagating some distance beyond anend face of the waveguide. Such a minimum phase fronts tend to occurabout 5 to 30 μm away from a waveguide end face, for the typicalwavelengths and mode sizes discussed herein. This enables substantialreduction in diffractive optical losses upon end coupling with anotherplanar waveguide or with an optical fiber. Multiple-core low-contrastwaveguides may be advantageously implemented where free-space opticalpropagation between waveguides is required, for example according to theteachings of earlier-cited application Ser. No. 10/682,768.Multiple-core low-contrast waveguides may be advantageously implementedwhere free-space optical propagation from the end face of the waveguideto a photodetector is required, for example according to the teachingsof earlier-cited application Ser. No. 10/661,709. Multiple-corelow-contrast waveguides may also exhibit reduced polarization and/orwavelength dependences relative to single-core planar waveguides. Whileremaining within the scope of the present disclosure and/or appendedclaims, the indices, thicknesses, and transverse dimensions for thecore, core layers, and cladding layers of any of the waveguides of FIGS.1A-1E, 2A-2E, 3A-3E, and 4A-4D may all be optimized to achieve desiredoperationally acceptable performance (with respect to optical loss,polarization dependence, wavelength dependence, spatial mode matching,and so forth).

The exemplary low-profile-core waveguides shown in the Figures representonly a sampling of various embodiments of planar waveguides that may beformed with one or more low-profile core(s) or core layers. Whileexemplary embodiments are shown that include one, two, three, or fourwaveguide cores or core layers, embodiments including still largernumbers of waveguide cores may be implemented within the scope of thepresent disclosure. Many other suitable low-profile-core waveguideconfigurations may be employed, and other suitable materials and/ormaterial combinations used therefor, while remaining within the scope ofthe present disclosure.

A low-contrast multiple-core waveguide as disclosed herein may exhibitrelatively little or substantially negligible polarization or wavelengthdependence. Such waveguides may be well-suited for applications wheresuch dependencies may be undesirable or unacceptable. For example, sucha substantially polarization-independent waveguide may be employed in anoptical receiver in which the polarization state of incoming light maynot be known or may vary over time. Such low-contrast multiple-corewaveguides may typically support modes more suitable for applicationsinvolving free-space optical propagation between adjacent end faces oftwo waveguides (often through an intervening optical component) thantheir high-contrast or single-core counterparts. The mode characteristicof a low-contrast multiple-core waveguide may suffer relatively lessdiffractive optical loss upon transmission between the waveguide endfaces.

As noted above, relative to single-core low-contrast waveguides (as inearlier-cited application Ser. No. 10/836,641), multiple-corelow-contrast waveguides disclosed herein tend to exhibit modalcharacteristics that are less dependent or negligibly dependent onpolarization or wavelength. In addition, for a given mode size(expressed as 1/e² HW power) in the vertical dimension, an optical modesupported by a single-core low-contrast waveguide has exponentiallydecaying wings that extend substantially farther from the mode axis thanthose of an optical mode supported by a multiple-core low-contrastwaveguide. As a result, for a given upper and lower cladding thickness,a multiple-core low-contrast waveguide exhibits less optical lossthrough coupling into the substrate or into the surroundings than asingle-core low-contrast waveguide. Or, for a given operationallyacceptable level of optical loss, thinner upper and lower claddinglayers may be employed with a multiple-core low-contrast waveguide.

Multiple-core low-contrast waveguides offer the fabrication advantagesof low-profile and thin cores (shallower etches, more preciselithography, substantially complete filling between etched features,substantially flat upper surfaces of deposited layers, and so on; asdisclosed in earlier-cited application Ser. Nos. 10/609,018 and10/836,641), while providing desirable optical properties characteristicof thicker single cores. For example, if the layers of FIGS. 1A-1E areeach deposited and spatially patterned sequentially, then no layerthicker than about 1.0 μm (and typically no thicker than about 0.7 μm)need ever be spatially patterned. The low-profile of patterned core 113,for example, ensures that subsequent deposition of a portion of middlecladding 120 b results in a substantially flat upper surface fordeposition of core layer 112. A multiple-core low-contrast waveguide asin FIGS. 1A-1E exhibits many of the desirable optical characteristics ofa thicker single core, but requires only a single patterning step of alayer typically less than about 1 μm thick, and often less than about0.7 μm thick.

The term “optical waveguide” (or equivalently, “waveguide”) as employedherein shall denote a structure adapted for supporting one or moreoptical modes. Such waveguides shall typically provide confinement of asupported optical mode in two transverse dimensions while allowingpropagation along a longitudinal dimension. The transverse andlongitudinal dimensions/directions shall be defined locally for a curvedwaveguide; the absolute orientations of the transverse and longitudinaldimensions may therefore vary along the length of a curvilinearwaveguide, for example. Examples of optical waveguides may include,without being limited to, various types of optical fiber and varioustypes of planar waveguides. The term “planar optical waveguide” (orequivalently, “planar waveguide”) as employed herein shall denote anyoptical waveguide that is provided on a substantially planar substrate.The longitudinal dimension (i.e., the propagation dimension) shall beconsidered substantially parallel to the substrate. A transversedimension substantially parallel to the substrate may be referred to asa lateral or horizontal dimension, while a transverse dimensionsubstantially perpendicular to the substrate may be referred to as avertical dimension. Terms such “above” and “below”, “top” and “bottom”,“up” and “down”, and so forth shall be defined relative to thesubstrate, with the waveguide defined as “above” the substrate. Examplesof such waveguides include ridge waveguides, buried waveguides,semiconductor waveguides (silicon, silicon-based, III-V, others), otherhigh-index waveguides (“high-index” being above about 2.5), silica-basedwaveguides (silica, doped silica, and/or other silica-based materials),polymer waveguides, other low-index waveguides (“low-index” being belowabout 2.5), core/clad type waveguides, multi-layer reflector (MLR)waveguides, metal-clad waveguides, air-guided waveguides, vacuum-guidedwaveguides, photonic crystal-based or photonic bandgap-based waveguides,waveguides incorporating electro-optic (EO) and/or electro-absorptive(EA) materials, waveguides incorporating non-linear-optical (NLO)materials, and myriad other examples not explicitly set forth hereinwhich may nevertheless fall within the scope of the present disclosureand/or appended claims. Many suitable substrate materials may beemployed, including semiconductor (silicon, silicon-based, III-V,others), crystalline, silica or silica-based, other glasses, ceramic,metal, and myriad other examples not explicitly set forth herein whichmay nevertheless fall within the scope of the present disclosure and/orappended claims. For purposes of the foregoing written descriptionand/or the appended claims, “index” may denote the bulk refractive indexof a particular material (also referred to herein as a “material index”)or may denote an “effective index” n_(eff), related to the propagationconstant β of a particular optical mode in a particular optical elementby β=2πn_(eff)/λ. The effective index may also be referred to herein asa “modal index”. “Low-contrast” or “low-index-contrast” shall denotematerials having an index contrast less than about 5%, while“high-contrast” or “high-index-contrast” shall denote materials havingan index contrast greater than about 5%.

One exemplary type of planar optical waveguide that may be suitable foruse with optical components disclosed herein is a so-called PLCwaveguide (Planar Lightwave Circuit). Such waveguides typically comprisesilica or silica-based waveguides (often ridge or buried waveguides;other waveguide configuration may also be employed) supported on asubstantially planar silicon substrate (often with an interposed silicaor silica-based optical buffer layer). Sets of one or more suchwaveguides may be referred to as planar waveguide circuits, opticalintegrated circuits, or opto-electronic integrated circuits. A PLCsubstrate with one or more PLC waveguides may be readily adapted formounting one or more optical sources, lasers, modulators, and/or otheroptical devices adapted for end-transfer of optical power with asuitably adapted PLC waveguide. A PLC substrate with one or more PLCwaveguides may be readily adapted (according to the teachings of U.S.Patent Application Pub. Nos. 2003/0081902, 2004/0052467, or2004/0264905, for example) for mounting one or more optical sources,lasers, modulators, photodetectors, and/or other optical devices adaptedfor transverse-transfer of optical power with a suitably adapted PLCwaveguide (mode-interference-coupled transverse-transfer orsubstantially adiabatic transverse-transfer; also referred to astransverse-coupling).

For purposes of the present written description or appended claims,“spatially-selective material processing techniques” shall encompassepitaxy, layer growth, lithography, photolithography, evaporativedeposition, sputtering, vapor deposition, chemical vapor deposition,beam deposition, beam-assisted deposition, ion beam deposition,ion-beam-assisted deposition, plasma-assisted deposition, wet etching,dry etching, ion etching (including reactive ion etching), ion milling,laser machining, spin deposition, spray-on deposition, electrochemicalplating or deposition, electroless plating, photo-resists, UV curing ordensification, micro-machining using precision saws or other mechanicalcutting/shaping tools, selective metallization or solder deposition,chemical-mechanical polishing for planarizing, any other suitablespatially-selective material processing techniques, combinationsthereof, or functional equivalents thereof. In particular, it should benoted that any step involving “spatially-selectively providing” or“spatial patterning” a layer or structure may involve either or both of:spatially-selective deposition or growth, or substantially uniformdeposition or growth (over a given area) followed by spatially-selectiveremoval (with or without intervening steps, which may or may not berelated to the patterning). Any spatially-selective deposition, removal,or other process may be a so-called direct-write process, or may be amasked process. It should be noted that any “layer” referred to hereinmay comprise a substantially homogeneous material layer, or may comprisean inhomogeneous set of one or more material sub-layers.Spatially-selective material processing techniques may be implemented ona wafer scale for simultaneous fabrication/processing of multiplestructures on a common substrate wafer.

It should be noted that various components, elements, structures, orlayers “secured to”, “connected to”, “deposited on”, “formed on”, or“positioned on” a substrate or layer may make direct contact with thesubstrate material or layer material, or may make contact with one ormore layer(s) or other intermediate structure(s) already present on thesubstrate or layer, and may therefore be indirectly “secured to”, etc,the substrate or layer.

The phrase “operationally acceptable” appears herein describing levelsof various performance parameters of optical components or opticaldevices, such as optical coupling coefficient (equivalently, opticalcoupling efficiency), optical throughput, undesirable optical modecoupling, optical loss, and so on. An operationally acceptable level maybe determined by any relevant set or subset of applicable constraints orrequirements arising from the performance, fabrication, device yield,assembly, testing, availability, cost, supply, demand, or other factorssurrounding the manufacture, deployment, or use of a particularassembled optical device. Such “operationally acceptable” levels of suchparameters may therefor vary within a given class of devices dependingon such constraints or requirements. For example, a lower opticalcoupling efficiency may be an acceptable trade-off for achieving lowerdevice fabrication costs in some instances, while higher opticalcoupling may be required in other instances in spite of higherfabrication costs. In another example, higher optical loss (due toscattering, absorption, undesirable optical coupling, and so on) may bean acceptable trade-off for achieving lower device fabrication cost orsmaller device size in some instances, while lower optical loss may berequired in other instances in spite of higher fabrication costs orlarger device size. Many other examples of such trade-offs may beimagined. Optical devices and fabrication methods therefor as disclosedherein, and equivalents thereof, may therefore be implemented withintolerances of varying precision depending on such “operationallyacceptable” constraints or requirements. Phrases such as “substantiallyadiabatic”, “substantially spatial-mode-matched”, “substantiallymodal-index-matched”, “so as to substantially avoid undesirable opticalcoupling”, and so on as used herein shall be construed in light of thisnotion of “operationally acceptable” performance.

For purposes of the present disclosure and appended claims, theconjunction “or” is to be construed inclusively (e.g., “a dog or a cat”would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat,or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or anytwo, or all three”), unless: i) it is explicitly stated otherwise, e.g.,by use of “either . . . or”, “only one of . . . ”, or similar language;or ii) two or more of the listed alternatives are mutually exclusivewithin the particular context, in which case “or” would encompass onlythose combinations involving non-mutually-exclusive alternatives.

While particular examples have been disclosed herein employing specificmaterials or material combinations and having particular dimensions andconfigurations, it should be understood that many materials or materialcombinations may be employed in any of a variety of dimensions orconfigurations while remaining within the scope of inventive conceptsdisclosed or claimed herein. It is intended that equivalents of thedisclosed exemplary embodiments and methods shall fall within the scopeof the present disclosure or appended claims. It is intended that thedisclosed exemplary embodiments and methods, and equivalents thereof,may be modified while remaining within the scope of the presentdisclosure or appended claims.

1. An planar optical waveguide, comprising: a substantially planarwaveguide substrate; a lower waveguide core layer; an upper waveguidecore layer; a waveguide core between the upper and lower waveguide corelayers; lower cladding between the substrate and the lower waveguidecore layer; upper cladding above the upper waveguide core layer; andmiddle cladding between the upper and lower waveguide core layerssubstantially surrounding the waveguide core, wherein: the lowercladding has a refractive index less than refractive indices of thelower waveguide core layer, the upper waveguide core layer, and thewaveguide core; the middle cladding has a refractive index less thanrefractive indices of the lower waveguide core layer, the upperwaveguide core layer, and the waveguide core; and the upper cladding hasa refractive index less than refractive indices of the lower waveguidecore layer, the upper waveguide core layer, and the waveguide core. 2.The apparatus of claim 1, wherein: a width of the waveguide core issubstantially larger than a thickness thereof; an upper surface of thewaveguide core is substantially flat; and the upper and lower waveguidecore layers extend bilaterally substantially beyond the lateral extentof a propagating optical mode supported by the planar optical waveguide,the supported optical mode being substantially confined bilaterally bythe waveguide core.
 3. The apparatus of claim 2, wherein an uppersurface of the middle cladding is substantially planar, and the upperwaveguide core layer is substantially planar.
 4. The apparatus of claim2, wherein: an upper surface of the middle cladding layer is non-planarand comprises a raised substantially flat portion above the waveguidecore; and the upper waveguide cladding layer is non-planar, andcomprises a raised substantially flat portion above the waveguide core.5. The apparatus of claim 2, further comprising a second waveguide corebetween the upper and lower waveguide core layers, wherein: a refractiveindex of the second waveguide core is greater than the refractiveindices of the upper cladding, the middle cladding, and the lowercladding; a width of the second waveguide core is substantially largerthan a thickness thereof; an upper surface of the second waveguide coreis substantially flat; and the first waveguide core and the secondwaveguide core are arranged one-above-the-other within the middlecladding.
 6. The apparatus of claim 5, wherein the waveguide cores arein contact with one another.
 7. The apparatus of claim 5, wherein: indexcontrast between the upper core layer and the upper and middle claddingsis less than about 5%; index contrast between the lower core layer andthe lower and middle claddings is less than about 5%; index contrastbetween the first waveguide core and the middle cladding is less thanabout 5%; and index contrast between the second waveguide core and themiddle cladding is greater than about 5%.
 8. The apparatus of claim 7,wherein the second waveguide core substantially confines a propagatingoptical mode along at least a portion of the length of the opticalwaveguide.
 9. The apparatus of claim 7, wherein: the upper, middle, andlower claddings comprise silica or doped silica; the upper and lowercores layers and the first waveguide core comprise doped silica; and thesecond waveguide core comprises silicon nitride or silicon oxynitride.10. The apparatus of claim 9, wherein: the upper and lower core layersare each between about 0.3 μm and about 2 μm thick; the first waveguidecore is between about 0.3 μm and about 1 μm thick and between about 3 μmand about 12 μm wide; the second waveguide core is less than about 2 μmwide and less than about 200 nm thick; the middle cladding between thelower core layer and the first waveguide core is between about 1 μm andabout 3 μm thick; and the middle cladding between the upper core layerand the first waveguide core is between about 1 μm and about 3 μm thick.11. The apparatus of claim 7, wherein: the second waveguide core tapersin at least one transverse dimension in a longitudinal direction along aportion of the waveguide; the second waveguide core tapers sufficientlygradually so as to substantially avoid optical coupling to unwantedoptical modes; and the apparatus functions so as to couple an opticalsignal between propagating optical modes respectively supported by thefirst and second waveguide cores.
 12. The apparatus of claim 2, furthercomprising an optical fiber or a second planar optical waveguide,wherein the planar optical waveguide terminates at a waveguide end face,the waveguide core and the upper and lower waveguide core layers eachreach the waveguide end face, and the planar optical waveguide isoptically end-coupled through the end face to the optical fiber or tothe second planar optical waveguide.
 13. The apparatus of claim 12,further comprising a tapered segment of the planar optical waveguidenear the waveguide end face wherein the upper and lower core layerstaper bilaterally along the terminal waveguide segment toward thewaveguide end face, wherein the supported optical mode is substantiallyconfined bilaterally at least in part by the upper and lower core layersat the waveguide end face.
 14. The apparatus of claim 13, wherein thetapered waveguide segment is bounded bilaterally by respectiveprotruding surfaces, and the upper and lower core layers and thecladdings are bounded bilaterally along the terminal segment of theplanar optical waveguide by the protruding surfaces.
 15. The apparatusof claim 14, wherein the protruding surfaces are substantially coveredby a substantially transparent embedding medium or encapsulant.
 16. Theapparatus of claim 13, wherein: the upper, middle, and lower claddingscomprise silica or doped silica; and the upper and lower core layers andthe waveguide core comprise doped silica.
 17. The apparatus of claim 16,wherein: the waveguide core is between about 0.3 μm and about 1 μm thickand between about 3 μm and about 12 μm wide at the waveguide end face;the upper and lower core layers are each between about 0.3 μm and about2 μm thick, are each wider than about the width of the waveguide core,and are each less than about 30 μm wider than the waveguide core at thewaveguide end face; the middle cladding between the lower core layer andthe first waveguide core is between about 1 μm and about 3 μm thick atthe waveguide end face; and the middle cladding between the upper corelayer and the first waveguide core is between about 1 μm and about 3 μmthick at the waveguide end face.
 18. The apparatus of claim 12, whereinend-coupled optical fiber or planar optical waveguide is longitudinallyspaced-apart from the waveguide end face, and diffractive opticalcoupling loss is less than about 0.3 dB for a longitudinal spacing fromthe waveguide end-face between about 10 μm and about 25 μm.
 19. Theapparatus of claim 2, wherein: index contrast between the upper corelayer and the upper and middle claddings is less than about 5%; indexcontrast between the lower core layer and the lower and middle claddingsis less than about 5%; and index contrast between the waveguide core andthe middle cladding is less than about 5%.
 20. The apparatus of claim19, wherein: the upper and lower waveguide core layers comprise dopedsilica between about 0.3 μm thick and about 2 μm thick; the waveguidecore comprises doped silica between about 0.3 μm thick and about 1 μmthick and between about 3 μm wide and about 12 μm wide; the lower,middle, and upper claddings comprise silica or doped silica; the middlecladding between the lower core layer and the first waveguide core isbetween about 1 μm and about 3 μm thick; and the middle cladding betweenthe upper core layer and the first waveguide core is between about 1 μmand about 3 μm thick.
 21. A method for forming a planar opticalwaveguide, comprising: forming a lower cladding layer on a waveguidesubstrate; forming a lower waveguide core layer on the lower claddinglayer; forming a lower portion of a middle cladding layer on the lowercore layer; forming a waveguide core on the lower portion of the middlecladding layer; forming an upper portion of the middle cladding layerover the waveguide core and on exposed areas of the lower portion of themiddle cladding layer; forming an upper waveguide core layer on theupper portion of the middle cladding layer; and forming an uppercladding layer on the upper waveguide core layer, wherein: the lowercladding has a refractive index less than refractive indices of thelower waveguide core layer, the upper waveguide core layer, and thewaveguide core; the middle cladding has a refractive index less thanrefractive indices of the lower waveguide core layer, the upperwaveguide core layer, and the waveguide core; and the upper cladding hasa refractive index less than refractive indices of the lower waveguidecore layer, the upper waveguide core layer, and the waveguide core. 22.The method of claim 21, wherein: a width of the waveguide core issubstantially larger than a thickness thereof; an upper surface of thewaveguide core is substantially flat; and the upper and lower waveguidecore layers extend bilaterally substantially beyond the lateral extentof a propagating optical mode supported by the planar optical waveguide,the supported optical mode being substantially confined bilaterally bythe waveguide core.
 23. The method of claim 22, further comprisingforming a second waveguide core over the first waveguide core, wherein:a refractive index of the second waveguide core is greater than therefractive indices of the upper cladding, the middle cladding, and thelower cladding; a width of the second waveguide core is substantiallylarger than a thickness thereof; an upper surface of the secondwaveguide core is substantially flat; and the first waveguide core andthe second waveguide core are arranged one-above-the-other within themiddle cladding.
 24. The method of claim 23, wherein the waveguide coresare in contact with one another.
 25. The method of claim 23, wherein:index contrast between the upper core layer and the upper and middlecladdings is less than about 5%; index contrast between the lower corelayer and the lower and middle claddings is less than about 5%; indexcontrast between the first waveguide core and the middle cladding isless than about 5%; and index contrast between the second waveguide coreand the middle cladding is greater than about 5%.
 26. The method ofclaim 25, wherein: the upper and lower core layers each comprise silicaor doped silica between about 0.3 μm and about 2 μm thick; the firstwaveguide core comprises doped silica and is between about 0.3 μm andabout 1 μm thick and between about 3 μm and about 12 μm wide; the secondwaveguide core comprises silicon nitride or silicon oxynitride and isless than about 2 μm wide and less than about 200 nm thick; the middlecladding comprises silica or doped silica; the middle cladding betweenthe lower core layer and the first waveguide core is between about 1 μmand about 3 μm thick; and the middle core between the upper core layerand the first waveguide core is between about 1 μm and about 3 μm thick.27. The method of claim 25, wherein: the second waveguide core tapers inat least one transverse dimension in a longitudinal direction along aportion of the waveguide; the second waveguide core tapers sufficientlygradually so as to substantially avoid optical coupling to unwantedoptical modes; and the apparatus functions so as to couple an opticalsignal between propagating optical modes respectively supported by thefirst and second waveguide cores.
 28. The method of claim 22, furthercomprising: forming a waveguide end face terminating the planar opticalwaveguide; and positioning an optical fiber or a second planar opticalwaveguide for optical end-coupling with the planar optical waveguidethrough the end face thereof, wherein the waveguide core and the upperand lower waveguide core layers each reach the waveguide end face. 29.The method of claim 28, further comprising forming a tapered segment ofthe planar optical waveguide near the waveguide end face wherein theupper and lower core layers taper bilaterally along the terminalwaveguide segment toward the waveguide end face, wherein the supportedoptical mode is substantially confined bilaterally at least in part bythe upper and lower core layers at the waveguide end face.
 30. Themethod of claim 29, further comprising forming bilateral protrudingsurfaces for bounding the terminal waveguide segment, wherein the upperand lower core layers and the claddings are bounded bilaterally alongthe tapered segment of the planar optical waveguide by the protrudingsurfaces.
 31. The method of claim 30, further comprising substantiallycovering the protruding surfaces with a substantially transparentembedding medium or encapsulant.
 32. The method of claim 29, wherein:the upper, middle, and lower claddings comprise silica or doped silica;the upper and lower cores layers and the waveguide core comprise dopedsilica; the waveguide core is between about 0.3 μm and about 1 μm thickand between about 3 μm and about 12 μm wide at the waveguide end face;the upper and lower core layers are each between about 0.3 μm and about2 μm thick, are each wider than about the width of the waveguide core,and are each less than about 30 μm wider than the waveguide core at thewaveguide end face; the middle cladding between the lower core layer andthe first waveguide core is between about 1 μm and about 3 μm thick atthe waveguide end face; and the middle cladding between the upper corelayer and the first waveguide core is between about 1 μm and about 3 μmthick at the waveguide end face.
 33. The method of claim 22, wherein:index contrast between the upper core layer and the upper and middlecladdings is less than about 5%; index contrast between the lower corelayer and the lower and middle claddings is less than about 5%; andindex contrast between the waveguide core and the middle cladding isless than about 5%.
 34. The method of claim 33, wherein: the upper andlower waveguide core layers comprise doped silica between about 0.3 μmthick and about 2 μm thick; the waveguide core comprises doped silicabetween about 0.3 μm thick and about 1 μm thick and between about 3 μmwide and about 12 μm wide; the lower, middle, and upper claddingscomprise silica or doped silica; the middle cladding between the lowercore layer and the first waveguide core is between about 1 μm and about3 μm thick; and the middle cladding between the upper core layer and thefirst waveguide core is between about 1 μm and about 3 μm thick.